The invention relates to an electronic appliance having an electrical circuit and an operating mode switching device which is associated with the latter and which can be used to wake the circuit from a quiescent state into an operating state in which the current draw of the circuit is greater than in the quiescent state, having a receiver for receiving a radio signal which has a carrier which has been subjected to amplitude modulation with a modulation signal, wherein the receiver has a UHF antenna which is connected to an input of a passive rectifier circuit via a passive impedance converter stage, wherein an output of the rectifier circuit is connected to a detection device for the modulation signal, and wherein the operating mode switching device has a control connection to the detection device for the purpose of waking the electrical circuit which is in the quiescent state.
An electronic appliance of this kind which is in the form of a battery-operated radio sensor node for a wireless sensor network and has a transceiver as an electrical circuit is known from Bas van der Doom et al. “A prototype low-cost wakeup radio for the 868 MHz band”, Int. J. Sensors Networks, vol. 5, no. 1, 2009, pages 22-32. Such an electronic appliance can easily be used to transmit data without complex wiring. A drawback in this case, however, is that the battery of the appliance has only a limited life. Networks in which the batteries in the appliances need to be replaced every month give rise to more costs in the long run than the one-off investment for the wiring of a corresponding wired network. This is the case particularly when the appliances are used at locations which can be accessed only with difficulty.
In order to reduce the power draw, the electrical circuit of the previously known appliance can be put into a quiescent state and an operating state by means of an operating mode switching device. In the quiescent state, the power draw is reduced in comparison with the operating state. By way of example, this can be achieved by disconnecting portions of the circuit in the quiescent state. What is known as waking the appliance takes place wirelessly using a radio signal which is emitted by a base station, for example. Said radio signal has a carrier in the 868 MHz band which has been subjected to OOK (on-off keying) amplitude modulation with a 125 kHz modulation signal. The OOK amplitude modulation involves the carrier being periodically switched on and off at a rate of 862 Hz. Since the carrier is in the UHF band, the appliance can be woken over a relatively long distance.
In order to receive the radio signal, the appliance has a receiver which has a UHF antenna which is connected to an input of a passive rectifier circuit via a passive impedance converter stage. The output of the rectifier circuit is connected to an interrupt input of a microcontroller via an operational amplifier, said microcontroller serving as a detection device for the modulation signal or the wakeup signal. The microcontroller is connected to an interrupt input of a microcomputer which controls the transceiver. Upon detection of the modulation signal, the microcomputer is woken via the interrupt input. However, the appliance still has a relatively high current draw even in the quiescent state. In particular, the operational amplifier, which needs to be in constant operation even in the quiescent state so that the appliance can be woken at any time, requires a relatively large amount of power.
It is therefore the object of providing an appliance of the type cited in the outset which allows a low current draw.